High temperature coating on ceramic substrate and non-firing process for obtaining same

Abstract

A process for producing a low porosity, high temperature coating on a porous substrate based on ceramic fibers of refractory compounds. The coating comprises from 30.8 to 41.9 weight % of silica (SiO 2 ) from a silica sol, from 30.2 to 49.4 weight % of alumina (Al 2 O 3 ), from 4.0 to 16.3 weight % of silicon carbide (SiC), and from 11.6 to 16.6 weight % of molybdenum disilicide (MoSi 2 ). The coating has an emissivity of not less than 0.8 and an operating temperature of up to 1500° C. The process is carried out without a firing step and includes the steps of preparing a slurry comprising from 65 to 75 weight % of a silica sol having a content of 20 to 50 weight % of SiO 2 and from 25 to 35 weight % of a mixture of particles of the refractory compounds Al 2 O 3 , SiC, and MoSi 2 ; applying the slurry to the ceramic fiber based porous substrate to form a layer of the slurry on the substrate; and drying the layer in several steps, first at ambient temperature, then at a temperature of 90° to 100° C., and then at a temperature of 200° to 220° C. until a constant weight of the coated substrate is obtained. The mixture contains from 52 to 71 weight % of Al 2 O 3 , from 6 to 28 weight % of SiC, and from 20 to 25 weight % of MoSi 2 .

What is claimed is: 
     
       1. A process for producing a low porosity, high temperature coating on a porous substrate based on ceramic fibers of refractory compounds, the coating comprising from 30.8 to 41.9 weight % of silica (SiO 2 ) from a silica sol, from 30.2 to 49.4 weight % of alumina (Al 2  O 3 ), from 4.0 to 16.3 weight % of silicon carbide (SiC), and from 11.6 to 16.6 weight % of molybdenum disilicide (MoSi 2 ), and having an emissivity of not less than 0.8 and an operating temperature of up to 1500° C., the process is carried out without a firing step and comprises the steps of: a) preparing a slurry comprising from 65 to 75 weight % of a silica sol having a content of 20 to 50 weight % of SiO 2  and from 25 to 35 weight % of a mixture of particles of the refractory compounds Al 2  O 3 , SiC, and MoSi 2 , the mixture having the following weight proportions:   ______________________________________                                    
Al.sub.2 O.sub.3 : 52 to 71%,                                             
SiC:                6 to 28%, and                                         
MoSi.sub.2 :       20 to 25%;                                             
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       b) applying the slurry to the ceramic fiber based porous substrate to form a layer of the slurry on the substrate; and   c) drying the layer in several steps, first at ambient temperature, then at a temperature of 90° to 100° C., and then at a temperature of 200° to 220° C. until a constant weight of the coated substrate is obtained.   
     
     
       2. The process according to claim 6, wherein the Al 2  O 3 , SiC and MoSi 2  components of the slurry are added in the form of whiskers, fibers, powders, or mixtures thereof. 
     
     
       3. The process according to claim 1, wherein the porous substrate is formed of a highly porous fibrous ceramic of silicon oxide, aluminum oxide, zirconium oxide, silicon carbide, silicon nitride, or a mixture thereof.